1. Field of the Invention
The present invention relates in general to a mono-block control valve used in hydraulic circuits and, more particularly, to a structural improvement in such a valve for substituting a side bypass passage for the typical center bypass passage, thereby improving the space efficiency of the valve and for connecting the right and left return passages to each other using the surplus space provided due to the side bypass passage and thereby reducing the passage resistance and achieving the stable operation of the valve.
2. Description of the Prior Art
In varieties of hydraulic power machines such as construction heavy equipment, the actuators are operated by pressurized oil delivered from a hydraulic pump. The actuators are thus provided with their control valves.
The mono-block control valve integrates the control valves for the actuators into a single body. The passages for connecting the control valves are formed in the single body of the mono-block control valve so that the mono-block control valve can simplify the valve structure, achieve the recent trend of compactness of the valves and remarkably improve the durability of the valves. Furthermore, as the mono-block control valve can be exclusively used differently from the separated control valves commonly used, the mono-block control valve has been widely used.
With reference to FIGS. 1 and 2, there is shown an example of the typical mono-block control valves. FIG. 1 is a longitudinally sectioned view of the control valve, while FIG. 2 is a cross sectioned view of the valve taken along the section line 2--2 of FIG. 1. As shown in the drawings, the mono-block control valve has a plurality of sections S1, S2 and S3, corresponding to control valves for the actuators respectively, in a single valve body B. When the spool (not shown) of a section, for example, the first section S1, is placed in its neutral position, the pressurized oil is transmitted to a next section, for example, the second section S2, through a first bridge 106a. The center of the valve body B is provided with a center bypass passage 101 which is used for returning the pressurized oil to a return tank T without resistance when the spools of all the sections S1, S2 and S3 are placed in their neutral positions. When at least one of the spools of the sections S1, S2 and S3 moves, the center bypass passage 101 will be shut down. The valve body B also includes a parallel passage 103 for supplying the pressurized oil to the sections S1, S2 and S3 in parallel. The passage 103 is formed in the body D such that the passage 103 passes all of the sections S1, S2 and S3. The pressurized oil delivered from the pump P is supplied to or discharged from the left and right actuator ports 104 and 104' from or to the parallel passage 103 through a second bridge 106b. The pressurized oil discharged from the left and right actuators ports 104 and 104' returns to the return tank T through return passages 105 and 105' formed in the opposite outermost portions of the valve body B.
However, the above mono-block control valve has a problem in that the valve B has five pump lines, which makes the passage Structure of the body B a complicated three-dimensional structure. Another problem of the above control valve is resided in that the valve necessarily has the bridges 106a and 106b, connecting the pump lines, and thereby enlarging the valve body size and increasing the cost.
The left and right return passages 105 and 105' passes the sections S1 and S2 and in turn are connected to each other through a connection passage 107 in the last section S3.
Therefore, the total length of the return passages 105 and 105' of the above mono-block control valve lengthens to increase the total passage resistance. In addition, there is a negative pressure difference between the left and right return passages 105 and 105' because the lengths of the return passages 105 and 105 extending from the actuator ports 104 and 104' to the return tank T are different from each other. For example, in the case of the first section S1, the oil discharged from the right actuator port 104' rapidly returns to the tank T simply through the return passage 105' with little resistance. However, the oil discharged from the left actuator port 104 returns to the return tank T through a longer passage. That is, the oil passes through return passage 105 extending in the sections S1 and S2 and in turn flows in the connection passage 107, making the two return passages 105 and 105' meet one another. Thereafter, the oil returns to the tank T through the return passage 105'. Such a difference between the lengths of the return passages can not help causing the negative pressure difference between the two return passages 105 and 105'. The above control valve also causes pressure loss and unstable operation because the negative pressure in a specified section having a relatively longer return passage is remarkably higher than those of the other sections.